With the growth of the wireless communications industry, battery-powered mobile terminals, such as mobile telephones, Personal Digital Assistants (PDAs), and the like, are becoming increasingly common. Since the mobile terminals are battery-powered, power consumption must be minimized. Such terminals often include an RF transmitter with an RF power amplifier as the last active stage in the RF transmitter chain. As a result, the RF power amplifier is typically the largest power consumption device in the mobile terminal; therefore, the RF power amplifier system must be as efficient as possible.
One commonly used technique for improving the efficiency of an RF power amplifier is to feed the direct current (DC) supply voltage of the RF power amplifier with a switching voltage regulator, such that the DC supply voltage is adjusted to allow the RF power amplifier to amplify RF signals properly and in an efficient manner. A switching voltage regulator could be used with an amplitude modulated (AM) transmitter by amplitude modulating the DC supply voltage for maximum efficiency. Such an AM transmitter could be part of a polar modulated RF transmitter. Polar modulated RF transmitters utilize both amplitude modulation and phase modulation (PM) to maximize the amount of information that can be encoded with minimum bandwidth.
A switching voltage regulator uses a switching device, such as a transistor, to connect and disconnect a system DC supply voltage to energy storage and filtering components, such as inductors or capacitors. By controlling the duty cycle of the switching device, the output voltage of the switching regulator can be regulated. The switching frequency of the switching device determines the frequency of ripple voltage on the output voltage. Often, a low drop-out (LDO) linear voltage regulator is used to minimize the effects of ripple voltage, and is connected in series with the output of the switching voltage regulator to provide the DC supply voltage to the RF power amplifier. The efficiency of such an arrangement is limited by the voltage drop across the LDO voltage regulator.
An AM switching voltage regulator must change its output voltage in proportion to an AM input signal. One technique is to use a switching voltage regulator with a series LDO voltage regulator. For the output voltage to be proportional to the AM input signal, the switching frequency of the switching voltage regulator must be larger than the frequency of the AM input signal, or the voltage drop across the LDO voltage regulator must be increased. This trade-off between increasing the switching frequency versus increasing the voltage drop across the LDO voltage regulator limits the efficiency of this technique. Thus, there is a need for a high efficiency AM switching voltage regulator.